a) Field of the Invention
The invention concerns a process and an apparatus for laminating a film of lithium by controlled detachment. More specifically, the invention concerns a process and an apparatus adapted to give thin films of lithium, characterized in that at the outlet of the rolling mill following a single pass, the film which has been laminated from a sheet of lithium, remains attached to the surface of one of the working rollers up to a given point on a part of the circumference of the latter working roller, past the meeting point of the two rollers, where it is drawn at a tension which is lower than the limit of elasticity of the film, but which is sufficient for detaching it from the surface of the one working roller, thereby obtaining an extra thin film of lithium and enabling to shape the film. The film of lithium obtained in this manner may be used as such in an electrochemical cell.
b) Description of Prior Art
In developing lithium batteries, one must rely on a process for manufacturing thin lithium electrodes in the form of continuous films. The films of lithium, which are commercially available, do not meet the norms of quality, length and width, and especially of thinness required for assembling a lithium battery with a polymer electrolyte. Since thin lithium has a very weak mechanical cohesion, it cannot undergo sufficient tension to keep its shape unmodified, as it is the case with conventional laminating processes using more resistant metals.
Traditional laminating processes, for metals such as steel or aluminum, utilize the cohesive strength of the latter to give desired inherent flatness and thickness. Thickness mainly results from the pressure exerted between the working rollers, by the holding tension applied to the metal at the inlet of the rolling mill and the unwinding speed, while shaping is essentially obtained by the profile of the rollers which are mechanically and thermally shaped, as well as by means of the tension applied on the metal at the outlet of the rolling mill. For those familiar with the handling of Li.degree., it is obvious that Li.degree. cannot undergo such physical stresses. To reach the desired thickness, one may use the pressure of the rollers and the speed. However, because of its very low mechanical cohesion, Li.degree. may only bear a minimal holding tension at the inlet of the rolling mill.
There is thus a need for shaping a film of Li.degree., since traditional laminating processes do not permit to produce an ultra thin film of lithium. As a matter of fact, high tensions may not be used with lithium, at the outlet of the rolling mills, because of its low limit of elasticity (579.13 KPa).
A survey of the prior art, on the other hand, teaches that there is no practical method for producing ultra thin films of lithium.
Thus, U.S. Pat. No. 3,721,113, inventor Hovsepian, dated Mar. 20, 1973, describes the lamination of a film of lithium between two working rollers whose surface is made of a polymeric material and wherein the surface tension does not exceed 46 dynes per centimeter at 20.degree. C.
U.S. Pat. No. 4,502,903, inventor Bruder, dated Mar. 5, 1985, describes a method of manufacturing a laminate of lithium on a conductive plastic by direct contact between a surface of lithium and a sheet of conductive plastic material.
U.S. Pat. No. 5,102,475, inventors Raynaud et al, describes the lamination of an alloy of magnesium lithium between two hard working rollers, to a thickness of 10 to 200 .mu.m. To overcome the problem caused by the adhesiveness of pure lithium on the rollers, an alloy of lithium and magnesium is used.
European document 0 146 236 published on Apr. 26, 1986, inventors Barry et al, describes an electrode which is protected by means a material which is impermeable towards fluids and which is non porous.
There are thus a certain number of patents such as described above which describe lamination of metals, however, in spite of this, thin lithium is not commercially available below 40.mu. and with substantial width and length such as is required for a polymer electrolyte battery, for example in the form of rolls 30 meters long by 15 centimeters wide and 22 microns thick.
It is thus seen that the prior art does not provide a process which permits to obtain in a continuous manner, ultra thin films of lithium, by lamination, especially in a single pass, and that in addition, it does not show how to control the inherent flatness of the lithium obtained.